David Krejci scored for Boston (28-17-7), which ended a 14-game losing streak (0-11-3) against the Capitals. Boston had not defeated Washington since March 29, 2014.
Rask (253 wins) passed Cecil "Tiny" Thompson. It was Rask's second shutout of the season and the 43rd in the NHL.
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"I enjoy it, obviously, but it doesn't change anything, really, in my mind during the season or even after the season," Rask said. "It's just your name is in the history books, that's about it. And then someday somebody else is going to come and break it, so hopefully I can put up a couple more and get some cushion."
Bruins coach Bruce Cassidy said, "He's done a great job for the Bruins organization for years, and his performance tonight was excellent. He was in control all night. We were pretty clean in front of him there till then end, and when we weren't, he was there for us."